1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display device, and particularly to a planarizing technique for an active-matrix substrate on which pixel electrodes and thin film transistors (TFT) for switching are integrally formed.
2. Description of the Related Art
A general construction of a prior art active-matrix liquid crystal display device will be briefly described with reference to FIG. 38. Each thin film transistor 3802 is integrally formed on the surface of a lower substrate 3810. A metal bus line pattern 3804 is electrically connected to a source region S of the thin film transistor 3802 through a first layer insulating film 3803. A pixel electrode 3806 is electrically connected to a drain region D of the thin film transistor 3802 through the first layer insulating film 3803 and a second layer insulating film 3805. The surface of the second layer insulating film 3805 is covered with an alignment film 3807. (Hereinafter, the lower substrate 3801 on which each thin film transistor 3802 and each pixel electrode 3806 are thus integrally formed is referred to as "active-matrix substrate" or "TFT substrate"). An upper substrate 3808 is disposed so as to face to the active-matrix substrate 3801 with a specified interval. A facing electrode 3809 and an alignment film 3810 are formed on the inner surface of the upper substrate 3808. (Hereinafter, such an upper facing substrate 3808 is referred to as "facing substrate"). A liquid crystal 3811 controlled in its alignment by the alignment films 3807 and 3810 is inserted in the interval between both the substrates 3801 and 3808. In the active-matrix liquid crystal display device having the above construction, when an image signal is supplied through the metal bus line pattern 3804 while a selection signal is applied to a gate electrode G of the thin film transistor 3802, a specified signal load is written on the pixel electrode 3806. The molecular alignment of the liquid crystal 3811 is changed depending on the voltage generated between the pixel electrode 3806 and the facing electrode 3810, to perform the desired image display.
Further, a general construction of a prior art active-matrix liquid crystal display device will be briefly described with reference to FIG. 41. The active-matrix liquid crystal display device has a cell structure including a pair of substrates 4101 and 4102 disposed so as to face to each other with a specified interval, and a liquid crystal layer 4103 held in the interval. One substrate 4101 is formed with pixel electrodes 4104 arranged in a matrix in the vertical and lateral directions, and switching devices connected to the individual pixel electrodes 4104. In this example, the switching devices are comprise thin film transistors (TFT). In each TFT, a drain electrode is connected to the corresponding pixel electrode 4104; a source electrode is connected to a signal line 4105; and a gate electrode is connected to a gate line 4106. (Hereinafter, the substrate 4101 having such a construction is referred to as "TFT substrate"). The other substrate 4102 includes a facing electrode 4107 for applying a vertical electric field to the liquid crystal layer 4103. In addition, the other substrate 4102 includes a color filter layer 4108. The color filter layer 4108 has segments divided into the primary colors, that is, red, green and blue. The segments are each matched with the pixel electrodes 4104. A pair of polarization plates 4109 and 4110 are stuck on both the surfaces of the cell structure.
The TFTs are selected in a line-sequential manner for each line through the gate line 4106, and an image signal is supplied to the TFTs through the signal line 4105, to perform the desired full color image display. The liquid crystal layer 4103 is, for example, in the twisted nematic alignment, so that the molecular alignment of the liquid crystal layer 4103 is changed in response to the vertical electric field applied between the facing electrode 4107 and each pixel electrode 4104. The change in the molecular alignment is taken off as the change in a transmitted light amount by a pair of the polarization plates 4109 and 4110, to thus perform the image display.
For the twisted nematic alignment of the liquid crystal layer 4103, the inner surfaces of a pair of the upper and lower substrates 4101 and 4102 must be subjected to an alignment treatment. The alignment treatment includes, for example, forming a specified alignment film and then rubbing the alignment film. FIG. 42 is a typical view showing the rubbing treatment. In this example, as shown in the figure, the inner surface of the lower TFT substrate 4101 is rubbed in the direction R, that is, bottom-to-top, while the inner surface of the facing substrate 4102 is rubbed in the direction R, that is, right-to-left. When a liquid crystal layer is sealingly inserted between a pair of the substrates 4101 and 4102 thus obtained, the liquid crystal molecules of the liquid crystal layer are twisted by 90.degree..
In the prior art structure shown in FIG. 38, wherein the thin film transistors 3802 and the metal bus line patterns 3804 are integrally formed on the active-matrix substrate 3801, the surface of the substrate 3801 is quite uneven, that is, has numerous irregularities and stepped portions. This makes it difficult to perform the alignment control for the liquid crystal 3811 and to obtain the uniform image display. The alignment of the liquid crystal is disturbed particularly in the stepped portions, which tends to cause the reverse tilt domain where the pretilt angle is reversed, thereby deteriorating the display quality. For shielding the region disturbed in the alignment, there has been known a technique of forming a black mask on the facing substrate side. The black mask is usually provided so as to be overlapped on the end portion of the pixel electrode liable to be disturbed in the alignment, thus sacrificing the effective display region. As a result, in aiming at enhancing the density of the pixel electrodes arranged in a matrix by reduction of the arrangement pitch thereof, the aperture ratio is lowered because the width of the black mask pattern cannot be reduced. Further, along with the reduction of the pixel pitch and the miniaturization of the chip size, the prior art structure has the following various disadvantages in terms of the manufacturing processes. For example, the thickness of the alignment film becomes uneven because of the large irregularities on the surface of the active-matrix substrate. This makes also difficult the uniform rubbing treatment for the alignment film. Further, when the active-matrix substrate is adhesively bonded with the facing substrate, there occurs a fault in adhesiveness because of the irregularities. Additionally, in the prior art structure, the direction of the electric field applied to the liquid crystal is made uneven by the effect of the irregularities on the surface of the active-matrix substrate, which obstructs the control to obtain uniform transmissivity. The liquid crystal is changed in its alignment depending on the electric field applied between each pixel electrode and the facing electrode, to be thus ON-OFF controlled. However, when the metal bus line and the gate line are raised around the pixel electrodes, the liquid crystal is affected by the lateral electric field. This disturbs the normal action by the synergistic effect with the disorder of the pretilt angle.
Along with the strong demands toward the fineness and accuracy in the active-matrix liquid crystal display device, the pixel pitch has been made small. To meet the above demands, it is required to enlarge the area of the pixel electrode as much as possible for ensuring the desired aperture ratio. Consequently, the interval between the adjacent pixel electrodes becomes smaller. In the extreme case, the interval between the adjacent pixel electrodes is made smaller than the interval between each pixel electrode and the facing electrode, so that the liquid crystal is occasionally affected by the subsidiary lateral electric field generated between the adjacent pixel electrodes larger than the normal vertical electric field applied between each pixel electrode and the facing electrode. In fact, the prior art structure has the following disadvantages: namely, by the effect of the lateral electric field, the reverse tilt domain is generated in the liquid crystal layer; and the light fallout is generated because the liquid crystal molecules are not correctly raised, resulting in the poor image quality.
FIG. 39 is a sectional view showing the construction of a prior art active-matrix liquid crystal display device. In this prior art structure, a pixel electrode 3906 is provided on a recessed portion surrounded by bus lines 3904 or the like arranged in a matrix. Accordingly, liquid crystal pixels are separated from each other. However, as the arrangement pitch is made fine along with the demands toward the high accuracy and fineness of the liquid crystal display device, the alignment defect of the liquid crystal 3903 is generated by the irregularities on the surface of the substrate. For example, when the substrate 3901 is rubbed in the direction of the arrow, liquid crystal molecules 3908 in the upper area of the pixel electrode 3906 have a specified pretilt angle, that is, are in the normal tilt state. In the areas near the tilt surfaces 3909 being the shady sides in the rubbing direction, the liquid crystal molecules 3908 are raised in the direction opposed to the normal tilt state, and are thus in the reverse tilt state. Consequently, a disclination is generated at the boundary between both the states, which deteriorates the display quality.
FIG. 40 is a typical view of the prior art structure shown in FIG. 39. As described above, since each pixel electrode 3906 is formed on a recessed portion surrounded by bus lines 3904, adjacent liquid crystal pixels are structurally separated from each other. However, it is difficult to apply the uniform rubbing treatment for the surface having the significant irregularities. In particular, as the arrangement pitch of the pixel electrodes is made fine along with the demands toward high accuracy and high fineness of the display device, the irregularities on the surface of the substrate is made relatively significant, tending to often cause the alignment defect.
Various means have been proposed to prevent the generation of the reverse tilt state. For example, Japanese Patent Laid-open No. HEI 4-305625 discloses a technique of forming grooves on a substrate for embedding thin film transistors and bus lines therein, thereby reducing the irregularities on the surface. Further, Japanese Patent Laid-open No. HEI 4-320212 discloses a technique of forming grooves in a layer insulating film for preventing the enlargement of the reverse tilt state. These techniques, however, fail to perfectly prevent the reverse tilt state.
FIG. 43 is a typical plan view of a TFT substrate. As shown in the figure, individual pixel electrodes are arranged in a matrix. They are repeatedly arranged in order of red, blue and green along the line direction. 0n the other hand, they are not linearly arranged in a row direction, and are shifted to each other by a half pitch in the lateral direction. As a result, the adjacent red, green and blue pixels are arranged in a triangular shape, that is, are in the delta arrangement, thus apparently improving the resolution. Each pixel electrode 4301 has an asymmetric shape in the right and left, and is provided with a portion to be matched with a contact hole C of a TFT (not shown).
In the liquid crystal display device, there is generally performed an A.C reverse drive, for example, the so-called 1H drive wherein the polarity of a signal voltage applied to pixel electrodes is reversed for each line. When the raster display is made by a 1H drive, for example, the center level of the signal voltage + the maximum signal voltage are applied to pixel electrodes in the first line; and the center level of the signal voltage-the maximum signal voltage are applied to pixel electrodes in the second line. Accordingly, a large potential difference .DELTA.V is generated between the vertically adjacent pixel electrodes 4301, which is twice as much as the maximum signal voltage. As the arrangement pitch between the pixel electrodes is made fine along with the demands toward the high accuracy and high fineness of the active-matrix liquid crystal display device, a lateral electric field intensity generated due to the above potential difference .DELTA.V cannot be neglected as compared with the vertical electric field intensity generated between the facing electric electrode and the pixel electrodes. In addition, the lateral electric field is generated in the plane direction of the figure, and the vertical electric field is generated in the direction vertical to the figure (in the thickness direction of the liquid crystal).
In the delta arrangement shown in FIG. 43, each pixel electrode 4301 has an asymmetric shape in the right and the left. Accordingly, between the pixel electrode in the first and second lines, the lateral electric field intensity is maximized at a region A, and minimized at a region B. Because of the unbalance of the lateral electric field intensities between the regions A and B, the liquid crystal molecules positioned on the lower portion of the pixel electrode in the first line are applied with a force F directed from the right to the left. On the other hand, as for the lateral electric field generated between the pixel electrodes generated in the second and the third lines, the above strong region A becomes weak, and the above weak region B becomes strong. Accordingly, the liquid crystal molecules positioned on the lower portion of the pixel electrodes in the second line is applied with a force F directed from the left to the right.
On the other hand, as described above with reference to FIG. 42, when the TFT substrate is rubbed in the direction R from the bottom to the top and the facing substrate is rubbed in the direction R from the right to the left, the rotational direction T of the liquid crystal molecules M becomes clockwise as seen from the facing substrate side in FIG. 43. In the first line, the liquid crystal molecules M are applied with the force F in the direction reversed to the rotational direction T, so that they tend to be rotated in the reversed direction against the dominating force of the vertical electric field, thus enlarging the so-called reverse tilt domain. On the other hand, in the second line, the liquid crystal molecules M are applied with a force F in the direction similar to the rotational direction T, so that they are raised rapidly in the normal direction, thus reducing the reverse tilt domain. As is apparent from the above description, for the pixel electrode having an asymmetric shape in the right and the left, by actually performing the 1H drive, the magnitudes of the reverse tilt domains become different.
FIG. 44 is a sectional view taken along the line Y--Y of the active-matrix liquid crystal display device of FIG. 43. From the left to the right in the figure, there are shown part of a pixel electrode in the first line, a pixel electrode in the second line, and part of a pixel electrode in the third line. As described above, a large reverse tilt domain region LRTD is generated between pixel electrodes in the first and second lines, and only a small reverse tilt domain region SRTD is generated between pixel electrodes in the second and the third lines. Since the reverse tilt domain regions reduce the display quality, they are generally shielded by black masks. As shown in the figure, a black mask 4411 is provided, for example, on the inner surface of a facing substrate 4402. The plane dimension of the black mask 4411 must be set to shield the large reverse tilt domain region LRTD. Accordingly, in the case that the reverse tilt domains are varied for each line due to the laterally asymmetric shape of each pixel electrode just as the prior art, the dimension of the black mask 4407 must be necessarily enlarged. This causes the disadvantage of sacrificing the aperture ratio of the active-matrix liquid crystal display device.